The present invention relates to a writing head in which a plurality of writing electrodes are arranged on a flexible support substrate and are disposed in contact with or in close proximity to a latent image carrier to supply writing voltages to the latent image carrier to form an electrostatic latent image thereon. The invention also relates to an image forming apparatus incorporating such a writing head.
In conventional image forming apparatus such as electrostatic copiers and printers, in general, the surface of a photosensitive body is charged uniformly by a charging device and the photosensitive body surface is exposed to light of an exposing device such as laser light, light of an LED lamp, or the like to form an electrostatic latent image thereon. The electrostatic latent image is developed by a developing device to form a toner image on the photosensitive body surface. The toner image is transferred to a medium such as a sheet of paper by a transferring device, whereby an image is formed on the transferring member.
In such conventional, general image forming apparatus are large and complex in structure because the exposing device for writing an electrostatic latent image is a device for emitting laser light or LED lamp light or the like. In view of this, an image forming apparatus has been proposed in which an electrostatic latent image is written to the surface of an image carrier by using writing electrodes instead of laser light or LED lamp light.
FIG. 36 is a perspective view of part of an example of such a writing head. The writing head 3 is composed of a flexible support substrate 3a, a plurality of wiring portions 3c (only two of them are shown in FIG. 36) that are a plurality of strip conductors arrayed on the support substrate 3a in the primary scanning direction of a latent image carrier 2, and writing electrodes 3b as projections that project toward the latent image carrier 2 from one ends of the respective wiring portions 3c. 
For example, the writing head 3 is formed by the following process. A conductor to be electrodes made of copper or the like is joined to an elastic and flexible insulative material to be a support substrate, and the conductor is coated with a photoresist. The photoresist is covered with a mask pattern corresponding to a wiring pattern and then exposure is performed. As a result, a writing head 3 is formed in which wiring portions 3c and writing electrodes 3b as rectangular parallelepiped or cubic projections that project from one ends of the respective wiring portions 3c are arranged on the support substrate 3a. 
In a writing head disclosed in Japanese Patent Publication No. 2002-172813A, a plurality of writing electrodes 3b are arrayed on a flexible support substrate 3a in the primary scanning direction in the above-described manner. Two arrays of writing electrodes 3b are arranged in a secondary scanning direction. And drivers are disposed on both sides of the writing electrodes 3b. 
In a latent image writing device disclosed in Japanese Patent Publication No. 2002-113897A, a plurality of writing electrodes are disposed in contact with or in close proximity to a latent image carrier in the above-described manner. A support substrate on which the writing electrodes are formed is pressed against the latent image carrier by a support member, a pressing member, and an urging member. This structure provides a large nip width with weak load.
It is also well-known that writing electrodes are arrayed in the axial direction of an image carrier. It is also well-known that a writing electrode formed on a flexed film-shaped substrate is brought into press contact with an image carrier with the aid of the elastic restoration force of the film-shaped substrate.
However, in the case where the writing electrodes are arrayed in the axial direction of the image carrier, current crosstalk may occur because of a small interval between the wiring portions of adjacent writing electrodes and it is difficult to increase the number of writing electrodes to enhance the resolution.
In the case where the two arrays of the writing electrodes are arranged in the secondary scanning direction, the crosstalk problem can be solved. However, it is difficult to assure high accuracy of positioning among the writing electrodes. Further, it is not suitable for downsizing because the writing electrodes occupy a large space, thereby increasing costs.
In the case where a writing electrode formed on a flexed film-shaped substrate, it is very difficult to stably bring the writing electrodes into contact with the image carrier because the elastic restoration force of the film-shaped substrate is unstable. Further, this method is not suitable for downsizing because the writing head occupies a large space.
In the above-described writing head that is composed of the flexible support substrate and the plural writing electrodes arrayed in the primary scanning direction, the rigidity is much lower in the portions between the electrodes or wiring patterns than in the electrodes or wiring patterns. Therefore, the writing head tends to wave or wrinkle in the primary scanning direction and hence it is difficult to stably bring the writing head into contact with the latent image carrier. As a result, an electrostatic latent image is not formed correctly on the latent image carrier, deteriorating the print quality.
In the writing head in which the two arrays of writing electrodes are arranged in the secondary scanning direction and the drivers are disposed on both sides of the support substrate, no wiring pattern exists in the region between the two arrays of writing electrodes and hence the rigidity is much lower there than in the other portions. Stress is concentrated in the low-rigidity portion and the writing head tends to be bent there: as in the above case, it is difficult to stably bring the two arrays of writing heads into contact with the latent image carrier. As a result, an electrostatic latent image is not formed correctly on the latent image carrier, deteriorating the print quality. If the writing head is bent, the distance between the two arrays of writing electrodes varied, resulting in a problem that disorder in the dot pitch of an electrostatic latent image causes horizontal streaks.